General antonyms:
* Uniform
* Solid
* Plain
* Even
* Homogeneous
* Unblemished
* Spotless
More specific antonyms depending on the context:
* Clear: If referring to color or surface (e.g., "clear skin" as opposed to "mottled skin")
* Smooth: If referring to texture (e.g., "smooth marble" as opposed to "mottled marble")
* Unmarked: If referring to a lack of patterns or blemishes
The best antonym will depend on the specific meaning of "mottled" in your context.